In many conventional lighting arrangements, a mechanical wall switch is used to turn ON or OFF a lighting unit by means of making or breaking an electrical connection between a load that includes the lighting unit, and a “hot” wire carrying power from the AC mains power source. Accordingly, the mechanical wall switch does not need a connection to the neutral wire from AC mains in order to turn ON and OFF the lighting unit, but instead only has an input terminal for being connected to the “hot” wire carrying power from the AC mains power source, and output terminal for supplying this power to the load when the switch turns ON the lighting unit (for safety reasons, the mechanical wall switch may also have a ground wire which does not supply any power to the wall switch or the load and which is connected to earth ground). As a result, in many existing buildings, the neutral wire from the AC mains power source is not provided to the junction box or other location where the mechanical wall switch is provided, but instead only the “hot” wire, and a wire to the load, are provided to this location (again, for safety reasons, a ground wire which does not supply any power to the wall switch or the load may also be provided and connected to earth ground).
Here it is understood that the load may include one or more lighting units, each of which may include a lighting driver and one or more light sources, such as an incandescent lamp, a fluorescent lamp (such as a compact fluorescent bulb), one or more light emitting diodes (LEDs). The load also may or may not include a ballast.
As energy saving requirements become more stringent together with the need for intelligent lighting systems, more and more electronic controllers which employ electronic switching and dimming capabilities are deployed in place of simple mechanical wall switches in residential and commercial installations. The operation of such an electronic controller is similar to that of a mechanical wall switch, but due to the electronic circuit inside the lighting controller the electronic controller may execute additional functions such as switching on or off a relay, dimming, wireless communications, etc. So, unlike a simple mechanical wall switch, the electronic lighting controller requires some energy for proper operation.
However, if the electronic controller is connected in place of a mechanical wall switch in front of the load, the maximum available power for the electronic controller is determined by the leakage current and the characteristics of the load, which is in series with the electronic controller. In some cases, for example those involving a dimming ballast whose leakage current is very limited, there is not a sufficient leakage current passing through the electronic controller when the load is turned OFF to keep the electronic switch operating properly. As a result, the lighting system may not operate properly.
FIG. 1 is a wiring diagram for a conventional lighting control system 100 which illustrates the issue. Lighting control system 100 includes a load 120 and an electronic controller 130.
Load 120 may include one or more lighting units and/or a motor (e.g., for a room fan). The lighting unit(s) may include lighting units each may include a lighting driver and one or more light sources, such as an incandescent lamp, a fluorescent lamp (such as a compact fluorescent bulb), one or more light emitting diodes (LEDs), etc. Load 120 also may or may not include a ballast. Load 120 has the first load terminal and a second load terminal, and is configured to receive a load voltage between the first and second load terminals and is further configured to allow a load current to flow between the first and second load terminals.
Electronic controller 130 has a single input terminal connected via a wire (e.g., a black wire) to a first power terminal 110 of an external power source 105 (e.g., AC mains) which outputs an AC voltage between first power terminal 110 and a second power terminal (e.g., a neutral terminal) 112 thereof. Also shown is a ground wire (e.g., a green wire) 112 which is connected to earth ground and which does not supply any power to the electronic controller 130 or load 120. Electronic controller 130 also has a single output terminal which is connected by a wire (e.g., a red wire) to the first load terminal of load 120. The second load terminal of load 120 is connected by a wire (e.g., a neutral wire, which may be a white wire) to neutral terminal 112 of external power source 105.
When electronic controller 130 is in an ON state so as to power load 120, then load 120 can receive as its load voltage 100% of the input voltage supplied from external power source 105. When electronic controller 130 is in an OFF state so as to disable load 120, then the load voltage across load 120 will be zero.
However, since electronic controller 130 is an electrical device which requires power to operate, the situation can become complicated. When electronic controller 130 is in the ON state, if the load voltage across load 120 is 100% of the input voltage supplied from external power source 105, then the voltage across electronic controller 130 will be zero, and it couldn't remain in the ON state for long. Meanwhile, when electronic controller 130 is in the OFF state, there will be no load voltage across load 120 and no load current flowing through load 120. However this means that there will also be no current, or very little current, passing through electronic controller 130, so it cannot maintain the OFF state, either if it requires more energy.
To address these issues, some electronic controllers are designed to modulate the time intervals when they are in the ON and OFF states. When the electronic controller is in the ON state, it will switch to the OFF state for a little while, (e.g., OFF for 2 ms during every 10 ms ON period), so that during this interval the electronic controller can receive 100% of the input voltage supplied from external power source 105 and thereby power itself. Meanwhile, when the electronic controller is in OFF state, it maintains a small leakage current flowing through the load, and with such leakage current, the electronic controller can power itself as well.
But along with the technology development and more and more features like wireless communication required for lighting control, the power consumption of an electronic controller increases significantly, and the intrinsic leakage current of the load itself is not sufficient to power the electronic controller when it is in the OFF state.
FIG. 2 is a wiring diagram for another lighting control system 200 which has been provided to try to address this issue. Lighting control system 200 is identical to lighting control system 100, except that lighting control system 200 includes an external capacitor 210 connected across the load terminals of load 120. Whether electronic controller is in an ON state or an OFF state, external capacitor 210 can provide a leakage current path for electronic controller 130. The bigger the capacitor, the more leakage current can be delivered to electronic controller 130 to support activities consuming much current and power (e.g., receiving a wireless control signal).
However, if electronic controller 120 includes a TRIAC based device, also known as leading edge dimmer, then external capacitor 210 will cause catastrophic damage to TRIAC in terms of huge inrush current every cycle. Additionally, external capacitor 210 will shift the phase of voltage and current at the load side, making the phase cutting of the dimming operation out of control.
Thus, it would be desirable to provide a lighting control system which can supply a necessary leakage current to a controller when the controller is in an OFF state and disables a load whose power is supplied by the controller.